This invention relates to a device which produces high intensity x-rays by bombarding a flowing stream of liquid gallium with an electron beam, thereby avoiding the cooling and contamination problems associated with prior art x-ray sources while permitting the use of large electron beam power.
The need for continuous duty, high intensity x-ray tubes exists in medical radiography, i.e., fluoroscopy and computerized tomography (CT), and in industrial applications such as x-ray diffraction and non-destructive testing. In the prior art the efficiency and lifetime of these instruments has been limited by two major factors: the tendency of large power densities of bombarding electron beams to heat the target surface, roughing and even melting the anode at the focal spot, and the buildup on the anode of contaminating materials.
Some prior art devices avoid the problems of heating by rapidly rotating the anode to bring cooler metal surfaces into the focal spot area, or by coating the anode with, e.g. ceramic oxides, to increase the thermal emission of the target. Liquid cooled rotating anode x-ray tubes are common. A hollow anode is rotated, and the heat generated by irradiation by the electron beam is transmitted to a heat exchange surface, typically the interior wall of the hollow anode. Generally the interior wall is roughed to increase turbulence, and a flow of liquid coolant, usually water, is passed into contact with the wall to remove the heat and cool the anode.
One invention (U.S. Pat. No. 3,719,847 issued to Webster, Mar. 6, 1973) uses a liquid metal with a relatively low boiling point as a coolant within the rotating anode. The heat created by the energy beam striking the target wall is sufficient to vaporize the liquid metal in contact with the inner surface of the wall at the point where the beam strikes the wall, and heat is drawn away from the wall by vaporization. As the anode is rotated, the vaporized metal is returned by centrifugal force into contact with and condenses on other portions of the inner surface of the wall, where it is again available to draw off heat through vaporization.
Maintenance problems associated with prior art x-ray sources sometimes cause as much as 50% downtime. Because most rotary anode x-ray sources require high speed anode rotation and high coolant pressures, an appreciable portion of the downtime for rotary anode x-ray sources is related to mechanical wear on rotating bearings and seals, and the need to reestablish a vacuum after vacuum enclosed parts are repaired.
The utility of prior art high intensity x-ray sources is limited as well by the tendency for contaminating materials to accumulate on the anode. Water bearing surfaces are subject to buildup due to corrossion. In addition, both rotary and stationary anodes become contaminated by the buildup of filament material transferred to the anode from the cathode. For example, if the cathode is made of tungsten, tungsten characteristic lines will appear in the output x-ray spectrum, which may not be desirable for the particular purpose of the instrument.
In most cases the response of the prior art has been simply to require that the instrument be shut down and cleaned. Alternative cleaning methods introduce apparatus for continuous cleaning using sputtering of the anode surface, thereby increasing the size and complexity of the x-ray device and making it more breakdown prone.
It is therefore a primary object of this invention to provide an apparatus which avoids the cooling and cleaning problems typically associated with prior art high intensity x-ray sources.
In the accomplishment of the foregoing object, it is another important object of this invention to provide an apparatus which permits the use of larger electron beam power and therefore increases the maximum intensity of x-rays obtainable when compared to standard rotary anode sources.
It is another important object of this invention to provide an apparatus which is characterized by high stability, long lifetime and low maintenance.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.